For clay soils there is a lot of good information from which you can derive Poisson's Ratio from SPT in papers by Malcolm Stroud accessible by googling "Stroud SPT". However deriving input parameters for a Plaxis model from such a crude test is not rational or sensible - you might as well just guess a number between 0.25 and 0.30
It is difficult to get some direct correlation between penetration resistance of soil with its poisson's ratio. I agree with Keith's viewpoint, that it is not rational to deduce Plaxis input model parameters from such crude technique. Assuming poisson's ratio around 0.3 might be an equally good guess.